Plastic containers are preferred for holding various liquid products, such as beverages and laundry detergents, because of their light weight and excellent impact resistance. These containers are typically prepared by blow molding a thermoplastic preform in a blow mold. A preferred thermoplastic is polyethylene terephthalate, hereinafter referred to as "PET", which has a high transparency, good gas-barrier property, a light weight, excellent impact resistance, and an appropriate rigidity.
To facilitate handling of large-size plastic containers, e.g., 2 liters or more, it is desirable to provide a handle. However, it is difficult to form a handle integrally with the container during the stretch blow-molding process because it complicates the preform and blow mold design and/or because of the limited stretch capabilities of the material. For example, thermoplastic polyesters such as PET strain-harden as they are stretched, making it difficult to form tight corners or deep grooves during blow molding.
Therefore, various other methods have been suggested for forming a handle. In one example, a separately formed handle is placed in the blow mold along with the preform, and then as the preform is expanded into a container the container becomes joined to the handle in the mold. Due to the strain hardening characteristic of PET, this technique results in inadequate handle engagement depth and poor resistance of the handle to disengagement. In another known method, a previously blown container is placed in an injection mold and a handle is injection molded onto the container. In both of these methods involving "in-mold" attachment of the handle and container, there is an increase in the expense of the equipment, the process time, and/or the level of defects.
It is an object of the present invention to avoid the expense and productivity loss of the prior known "in-mold" handle operations.
Another object is to enable the manufacture of a deep recess (i.e., tight corner or deep groove) in a strain-hardenable plastic container, such that a separate handle with an attachment member engaging the recess will not pop off the container during handling or drop impact.